In a mobile radio system, conforming, for example, to the Universal Mobile Telecommunication System (UMTS) standard, communication connections are set up via radio interfaces between mobile subscriber stations (UE: User Equipment) and stationary base stations (Node B). In order to support the mobility of a subscriber station, the subscriber station must continually carry out measurements with regard to a possible transition (handover) to another base station. These measurements are performed both in an idle state and in a connected state (connected mode). For this reason a mode referred to as compressed mode (CM) was introduced in UMTS in the FDD (Frequency Division Duplex) operating mode in order to enable a subscriber station to perform inter-frequency and inter-RAT (Radio Access Technology) measurements also during an existing connection to a dedicated channel (DCH) even without a second receiving device. With only one receiving device the subscriber station can perform e.g. handover measurements to a GSM radio access network (GSM: Global System for Mobile Telecommunication) during an existing connection. According to the present specification different types of compressed operating modes can be configured by the network. The first operating mode is referred to as “Uplink (UL) in CM only”. This operating mode for an uplink connection in compressed mode only is advantageous, for example, if a terminal or, as the case may be, a subscriber station is equipped with a second receiving device, but said subscriber station has to perform measurements in, for example, the GSM 1800 frequency band close to the UMTS frequency band on which an existing connection is being maintained to the first receiving device. In such a case a continuous transmission of the subscriber station on a dedicated UMTS channel (UE UMTS DCH Transmission) would cause strong interference with the GSM measurements which are performed using the second receiving device.
A second operating mode is used in the compressed operating mode for uplink and downlink connections and is referred to as UL/DL CM (DL: Downlink). This operating mode is used in order to be able to avoid the requirement for a second receiving device in the subscriber station and also for a second synthesizer. The third operating mode for downlink connections in compressed mode only, which is referred to as “DL in CM only”, can be used if a single receiving station with two synthesizers is used in order to perform, for example, inter-RAT measurements in the GSM 900 frequency band (GSM 900 Inter RAT Measurements).
FIG. 2 illustrates the principle of the compressed operating mode. Data is transmitted via a plurality of frames fr, whereby currently between one and a maximum of seven slots per frame fr can be occupied by the subscriber station for the purpose of performing the measurements. These timeslots can be situated either in the middle of the individual frame fr or be distributed over two frames fr. The transmit power P is increased in the compressed frame frc, thereby maintaining the quality of the connection constant. A compressed frame of this kind therefore consists of compressed data cd and a gap G. During the gap G the subscriber station can perform a measurement on other resources, in particular other frequencies. Which of the frames fr, frc are compressed is decided by the network or, as the case may be, communication system. Compressed frames frc can be specified periodically or also on request. The rate and type of the compressed frames frc are variable and depend, for example, on the type of measurements to be performed by the subscriber station. The structure of the compressed operating mode is assigned to a specific subscriber station, the structures generally being different between different subscriber stations of a plurality of subscriber stations within a cell. In the compressed operating mode data is therefore transmitted by way of frames fr, some of said frames frc having, as compressed frames frc, transmission gaps G in which no data is sent.
Also provided for a communication system of this kind are multimedia broadcast transmissions and what are called multicast services (MBMS: Multimedia Broadcast and Multicast Service), this being a service in which the base stations transmit information of general interest on a commonly used channel. This shared channel is monitored by a plurality of subscriber stations. The general information can be similar, for example, to teletext in television or to the content which is transmitted via DAB (Digital Audio Broadcasting), but also includes services such as multimedia. Such a service can be used, for example, to transmit news of goals in a football match to a plurality of subscriber stations over a single channel. During the transmission over this channel, however, a continuous transmission of data without transmission gaps is planned. During the time of the reception gaps on the receiving station side, no corresponding transmission gaps are provided on the side of the transmitting station with MBMS, with the result that a data loss occurs during the reception gaps.
With the introduction of MBMS for UMTS the problem therefore arises that the physical channel (S-CCPCH) that is used for MBMS does not support the compressed operating mode (CM). In the cases in which a subscriber station is in a dedicated connection state it is clear that at the instants in which the subscriber station performs measurements in the case of a compressed operating mode, data which is transmitted at this time in the corresponding S-CCPCH frames is lost. This results in a loss of MBMS data which is transmitted in continuous sequence over the channel S-CCPCH. In this case the amount of lost data is dependent on the length of the gaps, the frequency of the gaps and the number of active CM sequences in the subscriber station.
FIG. 3 shows an example of data structures such as are received by a subscriber station over a dedicated connection in the compressed operating mode via what is referred to as a DPCH channel (DPCH: Dedicated Connection in CM) in the top illustration. The bottom diagram shows which data is received or, as the case may be, not received by the subscriber station as receiver in the operating mode with reception gaps in the case of the reception of continuous data over such a broadcast channel S-CCPCH during the reception of MBMS. The subscriber station performs various measurements, for example a measurement of the received signal strength of GSM signals (GSM RSSI: Signal Strength Indicator). Further measurements are performed, for example, with regard to a base station identification code BSIC and with regard to inter-FDD frequencies. It is clear that during these measurement times data on the channel S-CCPCH is lost if the subscriber station has only a single receiver device and must therefore change the reception frequency.
A similar problem arises if a subscriber station is in a forward directed access channel state of a cell (Cell FACH (Forward Access Channel) state) in which the subscriber station is assigned a generally specified or a subdivided transport channel in the uplink direction, the random access channel (RACH) for example, which the subscriber station can use for the access procedure at any time. Access to the FACH of the cell is characterized in that the position of the subscriber station is known to the UMTS terrestrial radio access network (UTRAN: UMTS Terrestrial Radio Access Network) at cell level with regard to the cell in which the subscriber station last executed a cell update procedure. In this state no permanently dedicated channel is assigned to the subscriber station and measurements in accordance with the compressed operating mode are not required. Nonetheless the subscriber station must continuously monitor the FACH in the downlink direction and inter-frequency and inter-RAT measurements need to be performed periodically. The duration of the measurement cycle corresponds to the duration of the largest transmission time interval (TTI) on the channel S-CCPCH used for the broadcast messages or, as the case may be, MBMS which can be observed by the subscriber station, the measurement sequences taking place periodically every 2k transmission time intervals, where k=1, 2, 3 for 80 ms TTI conforming to current specifications. In the case of a TTI measurement duration of 80 ms as the longest duration there are measurement periods of 160 ms, 320 ms or 640 ms, according to the choice of k. For illustration purposes FIG. 4 shows an example of a subscriber station in what is referred to as the cell FACH state with k=2. Every 320 ms the subscriber station can interrupt the MBMS reception on the corresponding channel S-CCPCH in case inter-frequency and RAT measurements are required.
A disadvantage with all the methods is that in the case of the compressed operating mode a subscriber station can only incompletely receive continuously and successively sent data on a channel S-CCPCH. Various approaches to solving the problem are currently under discussion. One approach consists in a transmitter with knowledge of the gaps on the receiving subscriber station side simply interrupting the transmission of MBMS data during these times and performing a discontinuous transmission (DTX).
Another approach consists in the subscriber station on the receiver side attempting to reconstruct missing data, for example by performing a decoding using a forward error correction (FEC) technique, e.g. using turbo-decoding and interleaving methods known per se. However, these approaches are problematic, since the measurement gaps of different receiver-side subscriber stations which are located within a cell and receive and MBMS data are not aligned with one another in respect of time. The corresponding structures of the gaps are measurement-specific, i.e. dependent on the type of measurement which is to be performed by a subscriber station, in other words, for example, inter-frequency or inter-RAT measurements, while this also depends, for example, on the position of the subscriber station within the cell.
The preferred approach at the present time is for the subscriber station to perform inter-frequency or inter-RAT measurements during an MBMS reception using discontinuous reception (DRX). In this case the MBMS data is sent and transmitted without interruption, with an individual receiving subscriber station simply losing the MBMS data which was not received during the time that inter-frequency and inter-RAT measurements were being performed. Said subscriber station would have to attempt to reconstruct the missing data through the use of a forward error correction method.